As a tire has a geometry that exhibits symmetry of revolution about an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane, respectively. The expressions “radially”, “axially” and “circumferentially” mean “in the radial direction”, “in the axial direction” and “in the circumferential direction”, respectively.
In general, a tire comprises a crown comprising a tread intended to come into contact with the ground via a tread surface, said crown being connected by two sidewalls to two beads intended to come into contact with a rim.
The tread is intended to come into contact with the ground via a tread surface over a contact patch of circumferential length LC and axial width LA, which depends on the mechanical stresses of load and pressure that are applied to the tire. In the case of an aeroplane tire, it is usual to define the contact patch when the tire in the new state is inflated to its recommended nominal pressure and is subjected to a radial deflection equal to 32%. By definition, the circumferential length LC and the axial width LA of the contact patch are the circumferential length and the axial width, respectively, of the rectangle in which the contact patch, of more or less elliptical shape, is inscribed. The contact patch is the footprint of the tire when the tire in the new state is subjected to a radial deflection equal to 32% under the combined action of the vertical load and the inflation pressure. By definition, the radial deflection of a tire is its radial deformation, or relative variation in radial height, when the tire passes from an unladen inflated state to a statically loaded inflated state. It is defined by the ratio of the variation in radial height of the tire to half the difference between the outside diameter of the tire and the maximum diameter of the rim measured on the rim flange. The outside diameter of the tire is measured under static conditions in an unladen state inflated to the nominal pressure as recommended, for example, by the Tire and Rim Association or TRA.
The tread is a torus-shaped volume extending radially from a bottom surface to the tread surface over a radial height H, extending axially from a first tread edge to a second tread edge, and extending circumferentially around the entire periphery of the tire having a circumferential length C. The tread made of elastomeric material is the wearing portion of the tire. The bottom surface is a theoretical surface delimiting the maximum permissible degree of wear. When the level of wear reaches this bottom surface, it is recommended to withdraw the tire from service.
The tread is generally made up of raised elements extending radially outwards from the bottom surface, said raised elements being separated by voids. In the case of an aeroplane tire, the raised elements are usually circumferential ribs separated by circumferential voids referred to as circumferential grooves. A circumferential rib extends radially between the bottom surface and the tread surface, over a radial distance referred to as the radial height. It extends axially between two lateral faces, over an axial distance referred to as the axial width, the axial width being measured at the tread surface. Finally, it extends circumferentially and continuously over the entire periphery of the tire. By way of example, a tread of an aeroplane tire may comprise, symmetrically about the equatorial plane, passing through the middle of the tread and perpendicular to the axis of rotation, two axially external circumferential ribs or shoulder ribs, limited axially on the outside by one of the two edges of the tread and on the inside by a circumferential groove, two intermediate circumferential ribs and, at the centre of the tread, a central circumferential rib. A central or intermediate circumferential rib extends axially from a first circumferential groove to a second circumferential groove, while an axially external circumferential rib or shoulder rib extends axially from an edge of the tread to a circumferential groove. Thus, a tread comprises at least two shoulder ribs, each extending axially from a tread edge to a circumferential groove over an axial width LS.
Positioned radially on the inside of the tread is the crown reinforcement, which is the tire crown reinforcing structure. The crown reinforcement of an aeroplane tire generally comprises at least one crown reinforcing layer referred to as the crown layer. Each crown layer is made up of reinforcing elements coated in an elastomeric material, i.e. one based on natural or synthetic rubber, said mutually parallel reinforcing elements making an angle of between +20° and −20° with the circumferential direction. In an aeroplane tire, the reinforcing elements of a crown layer are generally arranged circumferentially in an undulating curve.
Among the crown layers, a distinction is made between the working layers that constitute the working reinforcement, usually comprising textile reinforcing elements, and the protective layers that constitute the protective reinforcement, usually comprising metal or textile reinforcing elements, and are arranged radially on the outside of the working reinforcement. The working layers govern the mechanical behaviour of the crown. The reinforcing elements of the working layers are usually cords made up of spun textile filaments, preferably made of aliphatic polyamides or of aromatic polyamides. The protective layers essentially protect the working layers from attack likely to spread through the tread radially towards the inside of the tire. The reinforcing elements of the protective layers may be either cords made up of metal threads or cords made up of spun textile filaments.
Aeroplane manufacturers are constantly concerned with passenger safety and, therefore, with reducing the risks of failure of their craft. Among potential failure modes, the partial or complete loss of the tread of a tire with which an aeroplane landing gear is equipped is a critical failure mode that occurs during aeroplane takeoff or landing phases.
This failure mode occurs, in particular, when the tire runs over a blunt object that might be present by chance on the runway. Bearing in mind the harsh conditions of use of an aeroplane tire, which are characterized by a high inflation pressure, a high static load, a high dynamic load and a high speed, the tread of the tire running over the blunt object causes damage to the tread, this generally resulting in the cutting of the tread and then in pieces of tread of varying geometric dimensions and mass being thrown up.
The pieces of tread may then either strike the structures of the aeroplane and lead to significant structural damage because of the mechanical energy stored up by said pieces, with this mechanical energy being higher, the higher the mass and speed at which the pieces are thrown up, or may enter the aeroplane engines and lead to problems with the operation of said engines, if these engines are unable to absorb the pieces of tread because they are too great in size.
Reinforcing the structures of the aeroplane in order to withstand potential impacts, particularly those of pieces of tread, has been considered. However, for the same materials, this solution entails increasing the mass of the structure, something which is detrimental as far as aeroplane performance is concerned, which is why increasingly lightweight structural materials are being used. Mechanically strengthening the structure does not, however, solve the problem of pieces being thrown into the engines.
Devices affording protection against pieces of tread being thrown up have also been considered. The document WO 2010012913 describes a protective panel, the external surface of which comprises a composite material, and which is mounted, via deformable components, on a support connected to the structure of the aeroplane. The deformable components, which are fixed to several support stiffening components and are perpendicular to the external surface of the protective panel, are designed to buckle under the effect of impacts by thrown up pieces of tread. The document WO 2010052447 describes a device that protects the engines of an aeroplane from thrown up tire tread debris. This device comprises a protective bar connected in a pivoting manner to the aeroplane main landing gear, the protective bar being able to move between a first and a second position. In the first position, the protective bar extends laterally across the mounted assembly consisting of the tire and of a wheel, in order to intercept possible paths of tread debris.
Devices for breaking up the tread with a view to minimizing the size of the pieces of tread and therefore to minimizing impacts with the aeroplane have also been described. The document U.S. Pat. No. 7,669,798 describes break-up means that are situated between the wheel and another part of the aeroplane and are able to break up into several pieces the bit of tread which has become detached from the tire and is being thrown up towards the other part of the aeroplane. These break-up means, such as a grating with blades that are able to cut up the material of the tread, are designed to disperse said pieces.
The above-described protective or break-up devices have the disadvantage of constituting additional structures, the additional masses of which are detrimental to the payload of the aeroplane.
Devices for breaking up the tread that are incorporated into the tire have also been proposed. The document WO 2013092578 describes an aeroplane tire comprising a tread-separation layer radially inside the tread and radially outside the crown reinforcement. The document WO 2013092581 describes an aeroplane tire, the tread of which comprises rows of cavities that open onto the tread, are parallel to one another, are inclined at an angle at least equal to 45° with respect to the circumferential direction of the tire and are distributed circumferentially over at least a part of the periphery of the tire. The document WO 2013092585 describes an aeroplane tire, the tread of which comprises independent cavities that are distributed axially over at least a part of the axial width of the tread and are distributed circumferentially over at least a part of the periphery of the tire.
When the tread is attacked by a blunt object, more particularly at a shoulder rib, accidental tread separation is limited to this shoulder rib. In this case, the shoulder rib is often cut in the form of a strip, a part of which remains integral with the tire and the free part of which is likely to strike the structure of the aeroplane on each revolution of the wheel.